Water-absorbing resin (super absorbent polymer or SAP) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is widely used typically in the form of powder having a particle diameter of 1 mm or less for absorbent applications including a hygiene product such as a disposable diaper and a sanitary napkin, an agricultural and horticultural water retaining agent, an industrial waterproofing agent and the like.
Water-absorbing resin is produced from many different monomers and hydrophilic polymers as raw materials. Among others, polyacrylic acid (salt)-based water-absorbing resin, which contains an acrylic acid and/or a salt thereof as a monomer, has high water absorption performance and is thus industrially produced in the largest amount.
Water-absorbing resin is produced as product in the form of particulates through various steps including polymerization, drying, crushing, classifying, surface crosslinking and the like (Non Patent Literature 1). As for said polymerization, the following have been proposed: aqueous solution polymerization, reversed phase suspension polymerization, spray polymerization, droplet polymerization and the like. In particular, the aqueous solution polymerization, which involves no use of dispersion medium, is excellent in terms of the physical properties of the obtained water-absorbing resin and its productivity.
As for aqueous solution polymerization, the following techniques have been proposed: a polymerization technique under adiabatic conditions (see Patent Literature 1), a polymerization technique while removing heat through heat transfer and water evaporation at a surface in contact of a polymerization belt and a hydrogel (see Patent Literature 2), a polymerization technique while removing heat through heat transfer and water evaporation at a surface in contact of an inner wall of a polymerization device and a hydrogel (see Patent Literatures 3 and 4), a polymerization technique while foaming or expansion of a hydrogel (see Patent Literatures 5 to 7), a polymerization technique while removing heat with a belt of a continuous belt polymerization device (see Patent Literature 8), and a polymerization technique while controlling a gas phase temperature (see Patent Literatures 9 and 10). Also proposed are: a polymerization technique under reduced pressure (see Patent Literatures 11 to 13) and a polymerization technique under increased pressure (see Patent Literatures 14 to 18). Further, a polymerization technique of collecting and recycling water and/or a monomer component volatilized during polymerization (see Patent Literatures 2 and 5 to 7).